High power semiconductor switch module

Abstract: A high power module (10) containing high power, high frequency semiconductor switching devices (18) and methods of operating the same provide high power and low inductance. The module (10) incorporates compositional, geometrical and electrical symmetry. The module (10) also includes short internal leads, special IC chip substrates, trimmable gate lead resistances, a special composite metal/ceramic baseplate and a special terminal conductor overlap.

Description: This invention relates to a high power semiconductor device module. A preferred embodiment relates to a power module containing a plurality of high voltage and high current semiconductor switching devices which are connected electrically in parallel and operated at high frequency. Insulated gate bipolar transistors (IGBTs) are an extremely attractive semiconductor device for power applications. They are more attractive than a power-type insulated gate field effect transistor (IGFET), which is popularly referred to as a MOSFET. An IGBT can handle both high voltages and high currents with small die size and with relatively low "on" resistance. In addition, an IGBT can be switched rapidly, making IGBTs potentially useful as switches in a three phase inverter for a high power alternating current motor application. On the other hand, the high current density capability and low "on" resistance of the IGBT also present new challenges. The possibility of device failure is aggravated when the IGBT is handling high power. By high power, we mean current densities above about 135 amps per square centimetre of active chip area, at hundreds of volts. By high frequency switching we mean on/off frequencies above about 18 kilohertz, for example 30 kilohertz. As might be expected, significant impedance, material and mechanical problems are encountered in handling such power at high frequencies and low resistances. This is especially true for a high power/high frequency module, in which several such IGBTs are connected electrically in parallel. Heretofore, the foregoing problems have been so difficult that not many high power/high frequency IGBT modules have been commercially manufactured. Those made thus far have been made in relatively low volume, such that each module could be individually specially crafted. The present invention seeks to provide an improved semiconductor device module. Electronic Design, vol. 41, no. 15 (22.07.1993), page 35, D.Maliniak, "Power package purges wire bonds and their inductance" describes a module in accordance with the preamble of Claim 1. A subassembly similar to those specified in claim 1 is also known from FR-A-2 452 178. JP-A-62-150871 describes a semiconductive device of a switching module having elements disposed on concentric circles in a geometrically symmetrical manner. EP-A-0427143 describes a semiconductor module having at least one half-bridge with two series-connected semiconductor switches. A high power/high frequency transistor module in accordance with the present invention is characterised over the Electronic Design paper by the features specified in the characterising portion of Claim 1. The invention seeks to provide a high frequency/high power linear-type and circular-type modules capable of being manufactured on a commercial production basis. By commercial production basis, it is meant production volumes such used in the automotive industry. The invention also seeks to provide high power/high switching frequency IGBT modules having a high efficiency and a high durability, but which are still economically manufacturable in automotive-type high volumes. Preferably, the module includes one or more active switching devices having a high degree of compositional, geometrical and electrical symmetry. In addition, electrical leads are preferably kept extremely short to reduce parasitic impedance effects. In single switch embodiment, input and output terminals are substantially concentric and have overlapping integral circumferential flanges. The flanges are for electrical connection to a plurality of IGBTs that are symmetrically circumferentially arrayed around said flanges, in close radial proximity to said flanges. In a dual switch embodiment, input and output terminals are two pairs of overlapping flat plate-like conductors. Each plate in each pair of overlapped plates has a linear edge registered with the other plate edge of the pair. The registered edges of one terminal pair is laterally disposed from and generally parallel to, the registered edges of the other pair. A separate parallel row of generally equally spaced IGBTs is disposed close to each pair of registered edges. A parallel row of input/output terminals is disposed equally between the edges of the plate pairs. A portion of each plate extends under only one terminal, which it is connected to that terminal. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a switching transistor substrate subassembly used in an embodiment of module; Figure 2 is an electrical schematic version of the switching transistor substrate subassembly shown in Figure 1; Figure 3 is a conceptual electrical schematic diagram of a single switch module containing several electrically paralleled substrate subassemblies of the type shown in Figures 1 and 2; Figure 4 shows the electrical schematic version of Figure 3, as is actually embodied in Figures 6-11; Figure 5 is an electrical schematic diagram of a three phase DC to AC inverter which uses six switching transistors or six of the single switch modules shown in Figures 6-11; Figure 6 is a plan view of a single switch module having coaxial input/output terminals and cover in place; Figure 7 is an elevational interior view of the module shown in Figure 6 in an early stage of assembly before the outer coaxial terminal and housing side walls and top are included, showing a circular array of switching transistors and gate circuiting. This is essentially a view along the line 7-7 of Figure 9; Figure 8 is a plan interior view of the module of Figure 7 in a later stage of assembly and includes an outer coaxial terminal, housing side walls and filamentary wire connectors; Figure 9 is a cross-sectional view along line 9-9 of Figure 6; Figure 10 is a fragmentary cross-sectional and elevational view along the line 10-10 of Figure 6; Figure 11 is an exploded isometric view of the module shown in Figures 3-4 and 6-11; Figure 12 is a conceptual electrical schematic diagram of a dual switch module containing two groups of mutually electrically paralleled substrate subassemblies of the type shown in Figures 1 and 2; Figure 13 shows the electrical schematic version of Figure 12, as it is actually embodied in Figures 14-20; Figure 14 is a plan view, part broken away, of a linear type dual switch module in an early stage of assembly and showing two linear groups of transistor substrates; Figure 15 is a plan view of the linear type dual switch module shown in Figure 14 in a following stage of assembly, where the two groups of substrates are connected to their respective terminal plates; Figure 16 is a plan view of the module of Figure 15 in a later stage of assembly, where a second spacer and two more terminal plates have been added; Figure 17 is a plan view of the module of Figure 16 with housing and wire bonds added; Figure 18 is a central sectional view through the width of the module of Figure 17 after addition of a housing cover; Figure 19 is a central sectional view through the length of the module of Figure 17 after addition of a housing cover; and Figure 20 is an exploded isometric view of the linear type dual switch module shown in Figures 12-20. General Considerations In the described embodiments important chemical, mechanical and electrical considerations have been combined in a very special way to obtain a high power/high frequency switch of low inductance. This special interdependency is not believed to be readily apparent or understood merely from a simple description of the switch assembly. The function and interdependency of switch parts has to be described as well. Two complex switch modules are to be described. To avoid repetition and to make their features and operation more readily understandable, important generic aspects of them shall be described first. As indicated above, this invention involves a semiconductor module for handling high power electrical currents at high frequencies. It also involves the making and the operation of such a device. The module of the preferred embodiment has one or more active switching devices and exhibits a high degree of compositional, geometrical and electrical symmetry. In addition, electrical leads between any power device in the module and input and output terminals for the module are preferably kept extremely short to reduce parasitic impedance effects in the module. Input and output conductors for any given semiconductor device in a module are preferably designed to have a similar impedance. Also, if the module has multiple devices connected electrically in parallel, all input leads are preferably of the same material and of the same configuration. This ensures equal, that is uniform, impedance among all input leads and all output leads and a similarity in impedance between input and output leads. Such uniformity and similarity in input and output leads are a part of the module compositional and geometrical symmetry contemplated herein. Input and output conductors usually will each have multiple parts. For example, there may be a special external portion adapted for special connection to an external bus, an intermediate portion adapted for connection to a semiconductor switching device and a portion which actually makes the connection. The latter portion would be one or more filamentary wires or contact bumps which interconnect one or more selected regions of a semiconductor chip with the intermediate portion of a particular conductor. Hence, the bonded filamentary wire or contact bump can be considered to be an inner end part of the conductor in the completed module. Compositional and geometrical symmetry is desired among all corresponding parts of all input conductors, among all corresponding parts of all output conductors and, to the extent similarity is practical, between input and output conductors. The benefit of compositional and geometrical symmetry in input and output conductors is uniformity in current flow to and from all paralleled devices in the module. This uniformity in input/output current flow inherently comprises an initial important part of electrical symmetry considered herein. A further extension of the foregoing geometrical symmetry provides a second part of the electrical symmetry considered herein. The shape and size of the input and output conductors, at least in their intermediate portions, allows at least these portions to be disposed close to one another. The type of close disposition considered herein is one in which the parts fit together or nest together, with a dielectric layer spacing them apart. Further, when the completed module is operated, such disposed, fitted or nested portions of the input and output conductors have an electrical current flow which is substantially parallel but opposite in direction. Outer portions can also be so disposed, for example in concentric terminals. In concentric terminals it may be said that the respective configurations are the same or similar, or that they are only complementary. In any event, their configurations are such as to provide another aspect of geometrical symmetry, which can be referred to as close parallel proximity. In the concentric terminal embodiment shown in the drawings, a single element forms the outer and intermediate portions of the input and output connections to all the devices in the module. A separate concentric element is used for each of the input and output, of course. Such a construction is advantageous in its simplicity and manufacturing cost. Nesting of the concentric terminal portions is simple but effective. Also, it is statistically quite durable because of its simplicity. Care should be taken to ensure that all chip-to-element electrical connections are symmetrical in placement and type to preserve impedance uniformity between the point of external connection to the terminal and the chip surface. This will probably require circumferential symmetrical disposition of module chips around the concentric input and output conductor elements. These extensions of geometric symmetry provide a second aspect of electrical symmetry as well. When the input and output conductor portions are placed in close parallel proximity, current flow in those portions is parallel but opposite in direction. The close proximity of the opposed current flows, allows the inductance of one current flow to at least partially negate and even substantially cancel inductance of the other current flow. Substantial reductions in inductance have been achieved using this effect. This inductance cancellation provides a second important part of electrical symmetry contemplated herein. It is recognised that providing substantial close parallel proximity in input and output connections to a given semiconductor chip is difficult. This is because at least one of the connections must be a multiplicity of filamentary wires, each of which forms an arch between its ends. The other connection most likely will be linear or rectilinear, as in a straight or bent tab or strip soldered to the rear side of the chip. One should preferably at least make the input and output connections to the chip parallel in one plane, for example having an input bonding wire oriented to extend in the same direction as an output tab or strip. In any event, the portions of the chip-to-conductor connections which cannot be made completely parallel should be made as short as is practical. This would favour use of contact bumps for the chip-to-conductor connection if practical way of using them in such an assembly can be found. Reference is now made to a third important aspect of electrical symmetry that should preferably be observed in a module. It involves device matching. All of the transistors and diodes used in our module should preferably be tested and sorted. Only matched transistors and diodes should preferably be used in the module. Not only that, only matched transistor/diode pairs on the substrate subassemblies are used. By this it is meant that after the transistors are paired with a diode and mounted on a substrate subassembly, such as substrate subassembly 10 in Figure 1, they are tested again. The substrates having similar operating characteristics are put in the same module. One particular operating characteristic which may be most important is maximum current level of the transistor as mounted on the substrate. Thus, the individual transistors/diode pairs in the module which are electrically connected in parallel should preferably have similar individual operating characteristics. Moreover, the closer it is wished to operate all of such paralleled devices at their maximum power ratings, the closer they should be matched in their maximum current level rating. Matching operating characteristics is important. If the impedance of one device is significantly less than that of other paralleled devices, one device tends to sink more and more current until the device fails. Such action can occur very rapidly. Analogously, if the transistors are not matched in their maximum current level ratings, one transistor at a lesser rating can cause all the other transistors to be operated at less than their maximum current rating. There is a supplemental facet to the third aspect of electrical symmetry. The supplemental facet is that the matched devices should preferably have matched cooling. Matched cooling is important in the preferred embodiment because a semiconductor device generates heat during operation. If the heat is not removed, the device will increase in temperature. If it increases in temperature, its operating impedance is reduced. This effect in turn generates still more heat, which can cause an avalanche effect to catastrophic device failure. By matched cooling, it is meant that chip operating temperatures remain matched, that is substantially equal, during even extended operation at maximum rated power. Hopefully, they even remain constant in temperature. Matched cooling can be achieved by equal cooling of the device chips. It can also be achieved by cooling each chip at a different rate, but at a rate which considerably greater than the rate at which heat is generated by the chip and which maintains the chip below a predetermined operating temperature. Matched cooling might be obtainable by symmetrically disposing the device chips on a substrate and cooling the entire substrate uniformly or at least in the substrate portion where the chips are disposed. Alternatively, one might choose selectively but uniformly to cool each substrate area where a chip is disposed. A fourth aspect of electrical symmetry is contemplated herein as well. The fourth aspect of electrical symmetry resides in providing a predetermined, usually matched, control signal to each of the paralleled semiconductor devices in the module. To achieve this goal in an insulated gate controlled semiconductor switching device, the gate control circuitry should preferably provide an identical control voltage to each device. It is recognised this is possible to do by design. However, for commercial manufacturing convenience, it is preferred to include at least one trimmable resistance in the gate circuit. In the preferred embodiments, a separate trimmable resistance is included in the gate circuitry for each paralleled device in the module. Each trimmable gate resistance can be trimmed after the gate circuit is assembled. In such instance the gate circuit can be made with considerably greater performance tolerance and at lesser cost. The trimmable gate resistance is trimmed to provide equal voltage and/or current to each paralleled semiconductor device in the module. Such balance or matching of gate control signal is still another factor preferred to preserve electrical symmetry in device operation. One may even want to include such a trimmable resistance in a package or module containing a single device to tailor the output of such a package or module to be like that of another package or module. The semiconductor switching devices contemplated for the high frequency modules are preferably and most importantly insulated gate bipolar transistors (IGBTs). IGBTs are capable of significantly higher current densities and switching speeds than insulated gate field effect transistors (IGFETs), which are popularly also referred to as MOSFETs. IGBTs have been the devices of first choice for high power applications. However, they have previously not been used extensively, especially for large volume applications, because of difficulties in suitably packaging them. One considerably difficult packaging problem was high package inductance. The preferred embodiment seeks to solve that problem and others as well, for example, by making it easier to use multiple IGBTs paralleled in a module as one switch. Thus, especially high current modules can be more readily commercially produced . A principle application is for switch modules to be used in an three phase inverter for an electric automobile. On the other hand, MOSFET or IGFET modules can also benefit from such modules. Nonetheless, for simplicity, the following description will focus on IGBT modules. As to the modules contemplated herein, even a module containing only one semiconductor switching transistor chip can benefit. However, as indicated above, the preferred embodiments are particularly beneficial for a module containing several switching transistor chips which are connected electrically in parallel. Such paralleled chips can form single or multiple switches. A single switch is schematically shown in Figures 3 and 4 and actually shown in concentric form in Figures 6 through 11. However, the preferred embodiments are even more beneficial to multiple switch modules. A high side/low side double switch is schematically shown in connection with Figures 12-13. A linear version of the such a high side/low side double switch is actually shown in Figures 14-20. A concentric version of a high side/low side double switch, has been designed and includes a segmented triaxial concentric terminal having coplanar terminal contact areas for all devices of both switches. A pre-testable substrate subassembly for each switching transistor chip is provided. Each substrate subassembly has generally parallel input and output conductor tabs. The tabs have generally coplanar end portions for connection to the generally coplanar contact areas of the triaxial terminal. The input and output tabs are respectively in low resistance electrical communication with input and output areas of a switching transistor chip mounted on the substrate subassembly. A five-layer substrate, of alternating copper and dielectric layers, permits formation of the co-planar input and output tabs on the resultant substrate subassembly. The individual substrate subassemblies are circumferentially arrayed around the triaxial terminal, analogous to the concentric single switch embodiment described more fully herein. DETAILED DESCRIPTION Referring to Figure 1 the substrate subassembly 10 shown comprises a 21 millimetre by 14.5 millimetre beryllium oxide plate or wafer 12 of about 1 millimetre in thickness. Beryllium oxide is a particularly effective dielectric material for use in this application. It has a high thermal transfer coefficient but has a thermal expansion coefficient similar to that of silicon. Aluminium oxide and aluminium nitride also generally match silicon in thermal expansion characteristics. However, beryllium oxide has a thermal transfer coefficient considerably higher. In such instance, wafer 12 can be made correspondingly thicker, which, in turn, reduces parasitic capacitance between the upper surface and lower surfaces of wafer 12 which are metallised, as hereinafter described. We also hereinafter describe the metallised wafer as soldered to the surface of an electrically conductive bottom plate of a housing. Regardless of the size of the housing or the number of switching transistors disposed in the housing, wafer 12 is only big enough to support one switching transistor and necessary associated circuitry, such as for thermal tracking or other symmetry purposes. This reduces the area of the metallised surfaces and further reduces parasitic capacitance. Upper copper foil plate 14 and a lower copper foil plate 16 are disposed on opposite faces of wafer 12. Copper foil plates 14 and 16 are each about 20 millimetres by 13.5 millimetres and are about 0.25 millimetres thick. They are secured to the opposite major surfaces of beryllium oxide wafer 12 by any acceptable technique, as for example, direct copper bonding. Direct copper bonding is a known and accepted practice in which copper oxide is used to bond a copper sheet to a ceramic substrate. Upper copper plate 14 has an integral generally rectangular extension 14a which is about 14 millimetres wide and about 13.3 millimetres long. Thus, it overhangs wafer 12 about 12.8 millimetres. The exposed portions of the copper foil on top of the wafer, that is excluding tab 14a, has a 6.35-10.2 micrometre silver coating thereon to enhance solderability. Extension 14a has a first row of holes 16a and a second row of holes 16b, which are mutually substantially parallel and also substantially parallel to the adjacent edge of the wafer 12. The holes 16a and 16b are about 0.9 millimetres in diameter and spaced apart on about 2 millimetre centres in each row. The centre line of the row of 16a holes is parallel to the edge of the wafer and is spaced about 2.24 millimetres from the edge of the wafer. The centreline of the row of 16a holes is parallel to the centreline of the row of holes 16b. The centrelines of both rows of holes 16a and 16b are spaced about 2.36 millimetres apart. The row of holes 16a forms a first line 17a of stress relief in tab 14a. Line 17a is thus a first easy-bend line in tab 14a. The row of holes 16b forms a second line 17b of stress relief in tab 14a. Line 17b is thus a second easy-bend line in tab 14a. As can be seen in Figure 1, tab 14a has two opposite right angle bends along bend lines 17a and 17b that form a step in tab 14a. This step facilitates connection of the substrate subassembly to the centre terminal member, as will become apparent below. A silicon semiconductor switching transistor 18, such as an insulated gate bipolar transistor (IGBT) or an insulated gate field effect transistor (MOSFET), is disposed on the exposed major surface of upper copper plate 14. A soft, fast silicon semiconductor diode (SFD) 20 is also disposed on the exposed major surface of upper copper plate 14, next to switching transistor 18. Such a device is not considered to be an insulated gate device or a high powered semiconductor switching transistor. A small plate 22 is also disposed on copper plate 14. Small plate 22 is present to facilitate bonding of an aluminium filamentary wire to copper plate 14. If the filamentary wire that is bonded is of aluminium, it is preferred that the small metal plate 22 have an aluminium outer surface. Small metal plate 22 can be of an aluminium/copper laminate with the copper side soldered face down on the outer surface of copper plate 14. On the other hand, it is recognised that small metal plate 22 could be of any material which is metallurgically compatible with the filamentary connecting wire used. Similarly, the lower surface of small metal plate 22 can be of any material compatible with the process used to bond plate 22 to upper copper plate 14. The filamentary bonding wires are shown in Figures 8 and 9. Figure 2 is an electrical schematic diagram of the subassembly illustrated in Figure 1. The diode chip 20 forms a blocking diode across the emitter collector terminals of the switching transistor 18. It is used to protect the switching transistor 18 by shunting current from temporarily reversed voltages which may occur in the system in which this switch is used. As can be seen from Figures 1 and 2, each switching transistor 18 is paired with a shunting diode 20. The transistor chip 18 and diode chip 20 are preferably made of substantially similar semiconductor material and by substantially similar processes so that they can have substantially similar performance characteristics, including change in initial characteristics with change in temperature. Each shunting diode 20 is disposed in close thermal proximity to its switching transistor 18 so that the pair would experience a similar temperature environment. This aids in providing more consistency in operation of the resulting switch. As indicated above, a plurality of pairs of such switching transistors 18 and shunting diodes 20 would normally be used in parallel for high power switches. In Figure 3 six such pairs are connected electrically in parallel to form a single switch module. Each gate lead for each switching transistor 18a-18f has a trimmable resistance 32a-32f in series with it. The trimmable resistance is also shown as reference numeral 32 in Figure 5. It's importance will hereinafter be explained. The schematic diagram of Figure 3 conceptually shows how the six switching transistors and their associated diodes, are connected electrically in parallel. Also, in order to make such a parallel circuit, only switching transistors having quite similar electrical performance characteristics should be used. In such instance, no one switching transistor will tend to "hog" current and cause an avalanche failure of the transistor and then the module. Similarly, care must be taken that all the paralleled transistors are mounted in the same manner so that they are more likely to be at the same operating temperature. It is not necessary for all the transistors in a multiple switching transistor module to be mounted on a common ceramic substrate since the substrate would become unduly large in area and complexity. Masking of the substrate for selective metallisation may be required. The large area would tend to increase parasitic capacitance between the top metallised surface of the ceramic substrate and a conductive backplate on which it will be disposed. The large area may also preclude use of beryllium oxide, even though its higher thermal transfer rating might allow a higher current rating and/or produce a lower parasitic capacitance in the resulting module. Increased complexity, that would be inherent to the large substrate, reduces yields of the large substrate and thereby increases its cost. In the preferred embodiments, each switching transistor is disposed on its own ceramic substrate. As indicated above, this is used to minimise individual and total size of the substrate. However, this provides another benefit. If the substrates are relatively small, substantially the entire substrate surfaces can be metallised, without appreciably increasing parasitic capacitance. This eliminates masking for metallisation. Elimination of masking further reduces costs. Yields of transistor groups will increase because imperfect transistor/substrate combinations or subassemblies can be discarded before the groups are formed. In addition, the mounting of a smaller substrate onto the housing bottom plate is easier and more readily fixable if the initial mount is not satisfactory. Thus, yields can increase still further, resulting in lower cost products and indirectly superior products. A corollary to the foregoing is that such cost reductions tend to make use of beryllium oxide even more practical for larger volume use. Hence the attendant increase in module performance becomes more practical for commercial production applications and improved products more available to the public. In addition, use of individual substrates for each switching transistor provides still another benefit, partly referred to above. Each substrate can be individually tested, graded and sorted after being made. Unsatisfactory chip/substrate combinations can be discarded before any more value is added to them, which affects cost. Grading transistors before grouping can be very important. However, unmounted transistor chips cannot be tested as thoroughly as mounted transistor chips. Accordingly, the mounted chips forming substrate assemblies 10, can be graded better and thus matched better. From a module performance standpoint, this can be very important. With better initial matching, the poorest performer in any group is still close to the best. Accordingly, even if the group must be operated at the power level of the poorest performer to prevent current "hogging", the group can be operated closer to the power level of the best. It follows that increased performance groups can be realised and/or that the average power level utilisation of all transistors used in groups can be increased. It is recognised that there are differences in length and an attendant variance in voltage drop, in the gate lead 30g to the respective switching transistors 18a-18f in a module. With the described embodiments, even slight differences in such voltage drop can be avoided. It is proposed to include a trimmable resistor 32a-32f in the gate lead between 30g and each of switching transistor 18a-18f, respectively. Each of the trimmable gate lead resistors allows the gate voltage supplied to each transistor to be the same. Thus, the "on" resistance characteristics of each switching transistor in the group comprising the resulting module is more likely to be similar. This helps make actual performance of the switching transistors match even more, after mount. The trimmable resistor may even be used to adjust operating level of the mounted and assembled group to the level of one switching transistor which results in a lesser performance characteristic than pre-testing indicates it should exhibit. Thus, the potential for current "hogging" in the group is reduced even further. Such a resistance can even be used to match the switches paired in an inverter, especially if only one or two transistors are used for each switch. This is one of the reasons that resistance 32 is shown in the switch 42 in Figure 5. Referring back to Figure 1, the surface of switching transistor 18 is divided into a multiplicity of aluminium electrode or contact areas, that is wire bond areas, to reduce emitter series resistance. In this example, there are eight such areas 24. They surround a smaller rectangular aluminium alloy gate electrode or contact area 26. In this example, chip 18 is an IGBT. The rear side of the chip 18 forms a collector region for the switching transistor 18, which region is in low resistance electrical communication with copper plate 14, such as by soldering or the like. The larger rectangular regions 24 on the upper surface of chip 18 form generally uniformly spaced emitter contacts on the IGBT surface. As indicated above, emitter contacts 24 surround the central gate electrode contact region 26. A single filamentary wire is adequate for gate lead contact because the gate lead does not carry much current. The emitter region of the diode chip 20 is its entire upper surface, which is metallised to have an electrode thereon 28 of aluminium alloy suitable for wire bonding. The lower surface of diode chip 20 is soldered to the upper surface of copper plate 14. Thus, the cathode of chip 20 and the collector of the switching transistor 18 are connected electrically in parallel, as shown in Figure 2. It can be seen in Figure 3 that the normal conceptual way of presenting such a circuit is to dispose the emitter lead of one device next to the collector lead of another, which is not done in this case. It should also be noticed that when one views such a circuit conceptually the transistors are aligned in a row for convenience of illustration, even if one wanted eventually to connect them to a coaxial bus. This suggests actually mounting them that way for simplicity. However, mounting and connecting them as conceptually shown leads to a poorer module. Figure 3 is only included in this description to make it easier to understand the more complex layout of devices and resistances which are in fact used. Figure 4 is the same circuit of Figure 3 but showing the actual layout of devices, resistances and conductors of the coaxial terminal module of Figures 6-11. Figure 3 shows IGBT switching transistors 18a-18f, which are respectively paired with soft fast shunting diodes 20a-20f. The switching transistors 18a-18f respectively have gate leads 30a-30f. Gate leads 30a-30f respectively have trimmable electrical resistances 32a-32f. As indicated above, the trimmable electrical resistances 32a-32f are included in each respective gate lead between gate conductor 30g and each of switching transistors 18a-18f. For an IGBT power transistor having a die size of about 9.8 millimetres by 9.8 millimetres, a trimmable resistor having a nominal printed resistance of about 4 ohms (trimmable to about 7.5 ohms) can be used. Gate leads 30a-30f, including their respective series resistors 32a-32f, are electrically connected in parallel to gate conductor or lead 32g. Gate conductor 32g leads to a gate terminal G, 30. Transistors 18a-18f have respectively collector leads 34a-34f. Transistors 18a-18f have respectively emitter leads 36a-36f. Collector leads 34a-34f are each connected to a common lead 34g, which connects electrically in parallel all of the collector leads 34a-34f. Similarly, emitter leads 36a-36f are each connected to a common lead 36g, which connects electrically in parallel all of emitter leads 36a-36f. Collector lead 34g is a low resistance communication with centre terminal 34. Emitter lead 36g is in the low resistance electrical communication with the coaxial emitter terminal 36. Two additional leads are shown to each of the transistors 18a-18f. These are leads used to determine potential of the collector and emitter leads to continuously monitor their operating characteristics during operation of the module. It is preferred to monitor principally deviations from predetermined values. We refer to these connections as "Kelvin" connections. Collector Kelvin leads 34ak-34fk are respectively connected to the collectors of transistors 18a-18f. Similarly, emitter Kelvin leads 36ak-36fk are connected to the emitter leads of transistors 18a-18f. Lead 34gk ties collector Kelvin leads 34ak-34fk together in an electrically parallel arrangement and places each of them in low resistance contact with the collector Kelvin terminal 36k. Similarly, emitter Kelvin lead 36gk connects electrically in parallel all emitter Kelvin leads 36ak-36fk and places them in low resistance electrical communication with emitter Kelvin terminal 34k. In Figure 4 the concentric terminals 34 and 36 respectively comprise output and input terminal posts for the module. The six transistor/diode pairs shown in Figure 3 are circumferentially symmetrically arrayed around the centre terminals. The array is as uniform as the structure appears to permit. It results in collector leads 34a-34f being of substantially the same length. Corresponding portions of emitter leads 36a-36f are also of substantially the same length. Gate leads 30a-30f, including their trimmable resistors 32a-32f, are all of about of the same length. One can consider that transistors 18a-18f, collector leads 34a-34f and emitter leads 36a-36f are radially arrayed about the coaxial centre terminals. Gate leads 30a-30f can be considered as radially arrayed too. Concentric gate lead 30g, concentric emitter Kelvin lead 36gk and concentric collector Kelvin lead 34gk surround the circle of transistor/diode pairs. These leads are respectively connected to module gate terminal 30, collector Kelvin terminal 34k and module emitter Kelvin terminal 36k. Gate leads 30a-30f are symmetrical and are each connected to concentric gate lead 30g. Collector Kelvin leads 34ak-34fk are each connected to concentric collector Kelvin lead 34gk. The emitter leads only have two Kelvin leads, 36ck and 36dk. They are connected to concentric emitter Kelvin lead 36gk at the bottom of Figure 4. Figure 5 shows an electrical schematic of a three-phase inverter circuit that converts direct current (DC) from a battery 38 to alternating current (AC) for use by a alternating current motor 40. As it can be seen, each of the three motor windings 40a, 40b and 40c is connected between a pair of switching transistors, or a pair of two groups of electrically paralleled switching transistors. Each transistor or group of such transistors, forming a single switch is indicated by reference numeral 42. Each such switch 42 can thus be the module represented in Figures 3 and 4 or more importantly a module made as herein specified. For clarity of illustration only one transistor/diode pair and its associated terminal resistance is shown in Figure 5 in the enlarged area. As indicated above, when IGBTs are used for such switches, they cannot be satisfactorily used by just turning them on and off once for each cycle they are to cover. These devises turn "on" hard, which produces a square wave, not a sine wave. To get appropriately configured sine wave output, IGBTs are turned "on" and "off" very rapidly, many times during each sine wave cycle, using increasing and then decreasing gate voltage and pulse width modulation. Turn on/turn off frequencies of 20,000-30,000 Hz are not unusual for automotive traction motor applications. As previously indicated, the parasitic capacitance associated with such rapid switching is so significant that it has heretofore prevented high power IGBTs from being extensively used. The described module reduces such parasitic capacitance to a low level and reliability and performance to a high level. Reference is now made to Figures 6-11, which show the actual module illustrated schematically in Figures 3-4 and indirectly contemplated in Figure 5. The module comprises a baseplate 44 on which is disposed a ring-like housing member 46 and a cover member 71. These three elements form a substantially closed chamber. Baseplate 44 is a rectangular plate about 2-4 millimetres thick, 108 millimetres wide and 124 millimetres long. It is preferably of a highly thermally conductive material that has a thermal expansion coefficient approaching that of silicon. One might think that metal would be preferred as a baseplate 44, because of its high thermal conductivity. However, most high thermal conductivity metals also have a relatively high coefficient of expansion. Silicon has a relatively low coefficient of expansion. Large differences in the coefficient of expansion are objectionable. The known metals having a thermal expansion coefficient close to silicon, such as fernico, Kovar, Invar and the like, also have relatively low thermal conductivity. Accordingly, it is preferred to use laminated or other types of composite materials for baseplate 44. They are more desirable for this application because they are designed to have good thermal expansion matching to silicon and relatively high thermal transfer properties. The most attractive of such composites for the described application are metal/ceramic composites. Since they include metal, they are generally electrically conductive, which it is believed is a strong asset. Such a composite baseplate offers significant benefits, even though the silicon chips are disposed on discrete ceramic substrates, as hereinafter described. Hence, metal is not recommended for baseplate 44 even though effective cooling is important, as hereinbefore mentioned. On the other hand it has been found that a metal/ceramic composite can be quite effectively used in the described application. Not only can it be made to have high thermal conductivity but this can also be relatively low in its rate of expansion. It is preferred to use a composite material which has a coefficient of expansion somewhat close to that of silicon. More precisely, however, the composite baseplate 44 should preferably closely match the coefficient of expansion of the composite ceramic substrate subassembly on which the silicon chip is directly supported. The substrate subassembly should preferably be quite similar to silicon but it is recognised this may not be an exact, precise match. If the substrate subassembly coefficient of expansion does not exactly match that of silicon, it is preferred that the thermal expansion coefficient of the baseplate 44 match that of the substrate subassembly, not the silicon. The substrate subassembly will hereinafter be more fully described. As indicated, we prefer that the substrate be electrically conductive and solderable. Many metal/composites are commercially and experimentally available. If not inherently solderable, they should be treated to make them solderable, at least in the areas where the substrate subassemblies are to be mounted. When a substrate subassembly such as substrate subassembly 10 is soldered to a larger conductive support, the effective area of the substrate's lower copper plate 16 is increased. This enlarges parasitic capacitance. In turn, if the larger conductive support is supported directly on a still larger area aluminium heat sink member, parasitic capacitance increases even more. Using dielectric materials to space these items apart, is usually not desirable because it usually reduces thermal transfer. Hence, increased dielectric thickness is usually not considered desirable. We have found one metal/ceramic composite that is particularly effective when used as baseplate 44. It is sold under the designation MCX-693 by the Lanxide Corporation of Newark Delaware, US. It has a coefficient of expansion of about 5-12 ppm per degree centigrade. We prefer 6 ppm per degree centigrade, which is almost exactly the same as that of the Cu/BeO/Cu sandwich used in substrate subassembly 10. This is close to that of silicon, which is about 3.2 ppm per degree centigrade. The MCX-693 material is essentially a combination of metal and ceramic in which the metal retains its identity sufficiently to provide high thermal conductivity. The ceramic retains its identity sufficiently to lower expansion coefficient but not thermal conductivity. We have found that the MCX-693 composite material additionally has fairly good mechanical strength. Thus it can serve as the baseplate itself, not as a support for it, which eliminates an additional thermal transfer interfaces. It forms a rugged module assembly. Further, it can be coated or plated to make its surface solderable. Moreover, the MCX-693 material has sufficient strength to allow it to be made as a hollow body. This allows the baseplate 44 itself also to function as a cooling member. In such instance, baseplate 44 need not be mounted on a heatsink, such as heatsink 62, for cooling. It need only be mounted on a mechanical support analogous to what might be used for supporting the heatsink. This not only eliminates costs due to an additional member in the resulting system but eliminates a heat transfer interface. Eliminating a heat transfer interface and its inherent losses improves cooling. Improved cooling, in turn, allows the switching transistors to be operated at higher power levels. Disposed on baseplate or backplate 44 is a plurality of the substrate subassemblies 10. They are symmetrically arranged around centre coaxial collector terminal 34 and emitter terminal 36 and soldered to baseplate 44. Also disposed on baseplate 44, surrounding substrate subassemblies 10, is a ring-like printed circuit board member 50. Ring-like circuit board element 50 is adhesively bonded to backplate 44 and supports gate conductor 30g, collector Kelvin conductor 34gk, emitter Kelvin conductor 36gk and trimmable cermet resistor chips 32a-32f. Ultrasonically bonded filamentary wires form the electrical connections between the foregoing conductors and terminals and the transistors and diodes on the substrate assemblies 10. They are shown in Figures 8 and 9. It should be noted that the ring-like circuit board element 50 can be of the typical FR-1 epoxy/glass circuit board material that supports a copper layer which is formed into a conductor pattern. However, it can also be a porcelainised steel substrate on which a thick film cermet conductor pattern is printed. Disposed on and adhesively bonded or soldered to the ring-like circuit board 50 are the six alumina chip resistors 32a-32f hereinbefore referred to. Each trimmable resistor chip is disposed a given predetermined distance away from the transistor gate electrode to which it is to be connected. Thus, every gate lead between every transistor and its associated chip resistor is the same length. Each alumina chip has a cermet thick film resistor block printed on its upper face and a thick film cermet conductor coating enveloping each end and overlapping onto the printed resistor block. If the enveloping end coating is solderable, it can be soldered directly to conductor 30g on circuit board 50. If not solderable, then the connection to 30g can be made by wire bonding after adhesively bonding the trimmable resistor chip to the circuit board 50. In any event, the trimmable resistor chip is affixed to circuit board 50 with its backside down, leaving the resistor-coated topside surface available for laser trimming. On the other hand, if circuit board 50 is of porcelainised steel, resistor chips would not necessarily be used. Instead the trimmable resistors 32a-32f would be integrally included in circuit board 50 and integrally interconnected with gate conductor 30g. Hence, a soldered or filamentary wire connection with 30g is also eliminated, along with discrete resistor chips. Nonetheless, the remaining discussion will describe the module as having discrete trimmable resistor chips. One end of each of the trimmable resistors 32a-32f is connected to the gate electrode 26 of its respective associated switching transistor. This is done by means of a filamentary wire (shown in Figures 8 and 9) between the gate electrode 26 and wire bond contact pad or electrode on the trimmable resistor chip. Trimmable resistors 32a-32f are also disposed so that they are accessible for trimming or re-trimming at the end of assembly but before the housing cover 48 is put on. It can be seen that each tab 14a of the respective substrate subassemblies 10 is welded to a circumferential flange on the inner coaxial terminal 34. Accordingly, inner coaxial terminal 34 forms the collector or output terminal of the module. Tab 14a can be soldered or otherwise bonded in a low electrical resistance connection to the circumferential flange 34a on terminal 34. We prefer to make terminal 34a of copper and tab 14a of copper. Rather than soldering them together, we prefer to weld them together by electron or laser beam welding. Substrate subassemblies 10 have to be attached to base plate 44 early in the assembly process because they are soldered. After they are tested, the lowermost central parts of the coaxial terminal assembly are affixed to the baseplate by adhesives. Tab 14 is bent up at bend line 17b in order to accommodate collector flange 34a. Because of bend line 17b, tab 14 can readily and safely be bent up after substrate subassembly 10 is soldered to baseplate 44. After terminal 34 is bonded to the board, tab 14 is bent over terminal flange 34a, at bend line 17b. Because of bend line 17b, tab 14 bends readily and safely and ends up somewhat parallel to the upper surface of flange 34a to which it is bonded. Advantage can be taken of this pre-orientation of tab 14 on flange 34a to clamp the two together, mask tab edges and electron or laser beam weld tab interior portions 52 to flange 34a. If electron or laser beam welding could not be done, hand soldering of tab 14 to flange 34a might have been required, which would have significantly increased cost. Reference is now made to Figures 8 and 9. Figure 8 shows the module in the next stage of manufacture, after the stage of manufacture illustrated in Figure 7. Figure 8 shows a cross-sectional view of the completed device. A generally cylindrical dielectric sleeve 54 surrounds terminal 34. Sleeve 54 has a circumferential outward flange 54a that overlies the outward circumferential flange 34a of terminal 34. Dielectric spacer 54 can be of any suitable electrically non-conductive material, such as plastics. Disposed on dielectric spacer 54 is the outer coaxial terminal 36, which also has an outward circumferential flange 36a disposed above the other circumferential flanges 34a and 54a. While terminal 36 is substantially cylindrical, it can be seen in Figures 6 and 8 that the central part of terminal 36 has two diametrically opposite enlargements 37. These enlargements are respectively bored and threaded at 37a to facilitate connection of a bus bar to terminal 36. The upper end of terminal 34 is axially bored and threaded for connection to a collector bus. Having opposed bus connection points 37a on terminal 36 provides increased uniformity in electrical resistance between the emitter bus and the substrate subassemblies 10. It should be noted that a compromise has been made regarding opposed enlargements 37a. They are present to increase current uniformity but their presence interferes with the wire bonding head for the filamentary connector wires. It is intended that all filamentary wires of corresponding elements, for example 36a-36f, be of the same length. As can be seen in Figure 8, the filamentary wires bonding wires closest to the terminal enlargements 37, that is wires 36a and 36d, have to be angled slightly differently from the rest. This would make them slightly longer than the rest. However, this can be accommodated by matching their length in the other wires. It should also be noted that the hexagonal periphery of flange 36a provides a straight line edge on flange 36a opposite each substrate subassembly 10. This allows the multiple wires in each group of IGBT emitter wires 36a-36f to be closer to the same length. In addition, it should be recognised that each group of filamentary connecting wires 36a-36f is generally parallel to tab 14a. However, they are parallel in only in one plane, are somewhat spaced apart and are of a different configuration from tab 14a. Inductance cancellation between them is therefore not optimum. The straight edges on terminal flange 36a allows the average length of the connecting wires 36a-36f to be shorter. Hence the length of uncancelled inductance is shorter, reducing module losses and increasing performance. It should be recognised that the filamentary wires 28a connect the emitter side of SFD chip 20 to flange 36a of input terminal 36. Filamentary wires 36a-36f respectively connect emitter areas 24 of transistors 18a-18f to flange 36a of input terminal 36. The filamentary wires and chip metallisations are preferably relatively thick aluminium alloy, effective to reduce series resistance. Electrode 26 on chips 18a-18f are respectively in low resistance electric communication with laser trimmable chip resistors 32a-32f by means of filamentary wires 26a-26f. One end of each filamentary wire 26a-26f is bonded to the electrode 26. The other end is bonded to the electrode on one end of the laser trimmable chip resistor. The electrode on the other end of each chip resistor is connected by a filamentary wire to a bond tab that is part of gate conductor pattern 30g on the patterned circuit board 50. An emitter Kelvin connection is made to flange 36a by a filamentary wire 36ak extending from flange 36 to a bond pad on circuit board 50, which bond pad is part of a collector Kelvin conductor 36gk on circuit board 50. Terminal 36 and in particular the upper surface of flange 36a is nickel-plated to enhance bonding of aluminium filamentary wires thereto. Trimmable chip resistors 32a-32f can be soldered directly to gate conductor 30g on board 50, as previously mentioned. If they are not, one end of a filamentary wire will have to be bonded to one end of the chip resistor and to the other to gate conductor 30g. In such instance, a wire bond pad is first soldered to the circuit board. The wire bond pads on board 50 and chips 32a-32f are of a material or at least have a surface coated with a material which is metallurgically compatible with the wire bonded thereto. An aluminium surface may be preferred if the filamentary wire is aluminium. The collector Kelvin connection is made by bonding filamentary wires 34ak-34fk respectively between the metal chip 22 on each substrate subassembly 10 and an adjacent bond pad that is part of Kelvin collector conductor 34gk on circuit board 50. Filamentary wires 34ak-34fk can be of aluminium and ultrasonically bonded. Also as hereinbefore indicated, the bond pad surface on chip 22 should be aluminium or of some other metal generally compatible with the bonding wire. Flange 34a on terminal 34 is electrically isolated from backplate 44 by a circular flanged element 56 having a circumferential flange 56a and a central tubular portion 56b. Flange 56 has boss 56c on its lower surface that cooperates with a corresponding recess in the upper surface of baseplate 44. This locates element 56 on baseplate 44. It also gives an added mechanical lock between them to prevent twisting of the coaxial terminal assembly when the bus bars are attached to it and bolted in place. Element 56 is adhesively bonded to the upper surface of backplate 44 and to the lower surface of flange 34a of terminal 34. Flange 56a has recesses 56b in its upper and lower surfaces. The recesses 56b induce a more uniform thickness of adhesive between element 56 and underlying baseplate 44 and overlying flange 34a. Element 56 can be of any suitable dielectric material as for example, a non-conductive plastics. It can be of the same material as the dielectric spacer 54. It should also be mentioned that spacer 54 is adhesively bonded to the upper surface of terminal 34 and its flange 34a and to the lower surface of outer terminal 36 and its flange 36a. An adhesive recess 54b is provided in the upper surface of flange 54a analogous to the recesses 56b provided in the upper and lower surfaces of flange portion 56a of lower spacer 56. The input and output terminals 34 and 36 and their associated spacers 54 and 56 have been described and shown as separate elements. They do not all have to be separate. It is conceivable that terminal elements 34 and 36 could be insert moulded into a metal/plastics terminal subassembly. In such a subassembly, plastics is moulded around terminals 34 and 36 but leaving their critical contact surfaces exposed. Thus, only electrical contact parts of them are exposed. Their other parts are embedded in a matrix of electrically non-conductive plastics, that serves the function of spacers 54 and 56. In such instance reliability and cost should be increased because the resultant subassembly only has one surface to be adhesively bonded. That one surface is the bottom of the terminal subassembly, which would be adhesively bonded to baseplate 44. The entire subassembly thus far described is surrounded by an electrically non-conductive plastics housing member 72, which includes embedded gate terminal 30, embedded Kelvin collector terminal 34k and embedded emitter Kelvin terminal 36k. These terminals are embedded because it is preferred to mould them into the housing member 72. In such instance, each of terminals 30, 34k and 36k is part of a single lead frame during moulding. The lead frame has web portions (not shown) that hold the terminals together for moulding. The web portions are not covered by the moulding compound and are removed after moulding, as is usual. Accordingly, the web portions are not shown in the drawings. Housing member 72 and baseplate 44 are rectangular in their external configuration even though they house concentric terminals and a circular arrangement of components. This provides the largest area of contact between baseplate 44 and the surface of its supporting member, for example heat sink 62, when many such housing assemblies are disposed on heat sink 62. Housing member 72 is adhesively bonded to the upper surface of baseplate 44. The adhesive is not shown. Recesses 72b and surrounding land areas, are provided in the lower surface of housing member 72, for adhesive thickness uniformity. As seen in Figures 6-8, housing member 72 is substantially a rectangular ring. Its bottom is closed by the baseplate 44. Its top is closed by a cover 71 having a rectangular periphery that nests in a groove on the top edge of the housing member 72. The groove is filled with a silicone adhesive (not shown) of the type previously described. One such adhesive is available from Dow Chemical Company of Midland, Michigan and is typical of silicone adhesives used for semiconductor devices and hybrid circuit devices. The use of a silicone filling is not new. It is commonly used as a filler and at least as a coating, in semiconductor device packages. It not only passivates the chips but the resin helps mechanically. Terminals 34 and 36 project up out of the module through a conforming recess in the cover 71, as shown. The silicone filling can be substantially completed before cover 71 is seated in the groove on the top of the housing 72 and then finished after the cover is seated. This may include a bead of resin filling the space between the terminal 36 and the centre aperture of the cover 71. As indicated above, the completed switch assembly is mounted on the upper surface of heatsink 62, probably with other such modules (not shown). It is likely to share the heatsink with other modules if it is part of the three-phase inverter circuit illustrated in Figure 5. Baseplate 44 of the module is fixed to the heatsink 62 by means of centre bolt 58 and corner bolts 70. As is typical of such installations, a layer of silicone grease is preferably disposed between the baseplate 44 and the heat sink 62 to ensure good thermal transfer between the facing surfaces even if they are not perfectly flat. Centre mounting bolt 58 is trapped within a centre recess within terminal 34. Bushing 64 is nested in a recess of the baseplate and encircles the threaded lower portion of 58. Bushing 64 has a shoulder on its upper periphery that locates a belleville spring washer 66, which also encircles the threaded lower portion of bolt 58. Heatsink 62 has threaded bores 68 for receiving bolts 58 and 70. The head of bolt 58 is frictionally retained within the upper portion 56b of tubular spacer 56 for shipment. The head of bolt 58 is adapted to cooperate with a tool such as an allen wrench through threaded central bore 60 in terminal 34. Bolt 58 would be pushed down from frictional engagement with the top 56b of spacer 56 into engagement with a threaded bore 68 in heat sink 62. The head of bolt 58 engages the spring washer 66. Spring washer 66 helps maintain a constant pressure between the baseplate 44 and heat sink 62 during thermal cycling, without inducing too much mechanical stress between them. It can be seen that the four corners of the housing are also bolted down. Figure 10 shows one of the corner bolts 70 and its encircling spring washer 66. As with bolt 58, the corner bolts 70 are disposed in recesses but only the spring washer 66 is trapped. The foregoing detailed description of Figures 2-11 are of a single switch module. In such a module all of the substrates 10 are connected to terminal 34. Hence, the collectors of all transistors 18 are tied together at terminal 34. In addition, the emitters of all transistors 18 are connected electrically in parallel by their connection to terminal 36. Accordingly, the switch comprised by the module of Figures 3-4 and 6-11 would be functionally equivalent to the switch 42 between nodes 34 and 36 in the circuit diagram of Figure 5. However, if possible and if appropriate matching can be arranged, it would be desirable to combine the switch between nodes 34 and 36 of Figure 5 with the switch between nodes 36 and 37 of Figure 5. Combining two single switches in one module is referred to herein as a double switch module. In such instance, the two switches would have a common collector/emitter terminal, for example at node 36 in Figure 5. A double switch is schematically illustrated in greater detail in Figures 12 and 13. A linear form of such a double or dual, switch is actually shown in Figures 14-20. However, the double or dual, switch can also be made into a circular form. Figure 12 shows the electrical schematic from a conceptual standpoint for ease of understanding. Figure 13 shows the same electrical schematic in the actual manner it takes form in the following Figures 14-20. Figures 12 and 13 each show six transistor/diode pairs. However, in comparing Figure 12 with Figure 3, one can see that in Figure 12 (the double switch), the six pairs of transistors and diodes are divided into two groups of three pairs each. In Figure 3, all six pairs are grouped together in one switch. In Figure 12, the six transistor/diode pairs are divided into two groups. The three pairs of each group are electrically in parallel and form one switch, analogous to all six pairs in Figure 3. Analogous to the single switch module, the transistors and diodes in each group are initially tested and sorted before mounting on their discrete substrates. After mounting, each transistor/diode pair is tested and sorted as a pair. Similar pairs are then matched and placed in three member groups. Accordingly, the output characteristics of each substrate, that is mounted pair, in each group will be substantially the same to provide the symmetry discussed in connection with the single switch of Figures 3-4 and 6-11. In addition, it is highly preferred that all the transistors in both groups be the same to extend the symmetry even further. In Figure 12, the Group I transistors 18a, 18b and 18c all have the same collector terminal 34, emitter terminal 36, gate terminal 30, collector Kelvin 34k and emitter Kelvin 36k. The Group II transistors 18d, 18e and 18f have collector leads 36d, 36e and 36f. It can been seen that the Group II collector leads 74d, 74e and 74f are connected to the same bus 36g/76g as the Group I emitter leads 36a, 36b and 36c. Accordingly, terminal 36/74 functions as an emitter terminal for the Group I transistors and as a collector terminal for the Group II transistors. The emitter leads 76d, 76e and 76f for the Group II transistors have their own bus 76g, which is connected to the Group II transistor emitter terminal 76. Emitter terminal 76 is labelled E2 in Figures 12 and 13. Since it is intended that the transistors in Group I will be in the "on" condition when the transistors in Group II are in the "off" condition and vice versa, a separate gate control electrode is needed for each group. Accordingly, the Group II transistors have their own gate terminal 78, gate bus 78g and gate leads 78d, 78e and 78f. In order to match the transistors in each group, each of them has a laser trimmable variable resistance 32d, 32e and 32f in series with their respective gate conductors. Similarly, separate emitter and collector Kelvin electrodes 76k and 74k are provided for the Group II electrodes. Reference is now made to Figure 13, which schematically shows the actual module layout, including disposition of transistors, as shown in Figures 14-20. The principles of chemical, mechanical and electrical symmetry can also be applied to a linear structure, as for example the module of Figures 12-20. Moreover, they can be use to form a low inductance module that contains more than one switch. Figures 12-20 show a structure with only one pair of switches. On the other hand, is expected that this principle can be used to form low inductance switching modules with more than one pair of switches. Figures 12-20 are included in this description to illustrate that the important principles of chemical, mechanical and geometrical symmetry simplifies module construction sufficiently to make it commercially manufacturable. This is particularly true when the module is made using the described substrate. To emphasise both the similarities and the differences between the circular single switch of Figures 3-4 and 6-11 and the linear double switch of Figures 12-20 similar components are given similar reference numerals. In Figures 14-17, the three aligned substrate subassemblies 10 at the top of each figure form a first switch, while the three aligned subassemblies 10 at the bottom of each figure comprise the second switch. In the linear construction shown in Figures 14-20, the aforementioned input and output terminal parallelism and overlap are extensively used. For example, input and output conductors 34g and 36g for the first switch take the form of generally registered overlapping parallel plates (shown at the top of each of Figures 14-17). Similarly, input and output conductors for the second switch take the form of substantially registered overlapping parallel plates 74g and 76g (shown at the bottom of each of Figures 14-20. Because of such symmetry and overlap, input and output inductances in both switches are effectively cancelled or neutralised. Reference is now made to Figure 14, which shows a linear type, double switch module in an early stage of assembly. The module has a generally annular baseplate 80, which has a flat annular upper surface 82, that has a generally rectangular outer periphery. Baseplate 80 has an aperture 81 in each of its four corners to receive mounting bolts (not shown). The mounting bolts are used to clamp baseplate 80 to a support (not shown). As can better be seen in Figures 18-19, baseplate 80 is hollow and liquid cooled. Its support thus does not have to be a heat sink, such as heat sink 62 in the first-described embodiment. More will be said about liquid cooling of the baseplate later, in connection with the description of Figures 18 and 19. The inner periphery of baseplate surface 82 is formed by an elongated central slot 86. Slot 86 has two parallel long sides 86a and 86b and is symmetrically rounded at its opposed ends 86c, similar to the outer periphery of a horse race track. This results in two parallel elongated portions 82a and 82b of surface 82, spaced symmetrically by the slot. Each of the elongated surface portions 82a and 82b has an elongated rectangular region, respectively designated as 84a and 84b. Elongated rectangular regions 84a and 84b are symmetrically disposed with respect to one another on parallel surface portions 82a and 82b. Hence, the rate of cooling in corresponding parts will be similar. In addition, surface portions 84a and 84b are especially prepared to receive substrate subassemblies 10 and thick film resistor chips 32a-32f. By specially prepared, it is meant that regions 84a and 84b are readily solderable, that is nickel plated, if substrates 10 and/or chip resistor substrates 32a-32f are to be soldered in place. On the other hand, if they are to be adhesively bonded in place, baseplate surface portions 84a and 84b are especially prepared, that is etched and/or micro-roughened to aid in adhesion. The substrates 10 and chip resistor substrates 32a-32f are preferably the same as described in the preceding example. This would include a metallised lower surface that permits soldering to the baseplate surface portions 84a and 84b as shown. Chip resistor substrates 32a-32f would each be of ceramic, having a thick film cermet resistor coating in the middle of its upper surface and a connecting thick film cermet noble metal coating on each end. Substrates 10 would be as described earlier and shown in Figure 1. However, tab 14a is not shown in Figure 14 because it is of thin metal and is bent straight up in this view, perpendicular to surface 82. The bend is shown to be precisely at the edge of the substrate in Figures 14-20. On the other hand, it need not be that way. One may prefer the first bend to be more like that shown in Figure 1. Three substrates 10 are soldered to surface portion 84a and three more to surface portion 84b. Each group of three is along a line parallel to the centreline of slot 86 and spaced the same distance from it. The chip resistors 32a-32f are disposed as shown. Preferably they are also disposed symmetrically to obtain uniformity in cooling and thereby uniformity in temperature during operation. Maintaining such uniformity in temperature is important because resistors can change resistance with temperature. In this module resistor chips 32a-32f are present to provide uniform gate voltage to the transistors on substrates 10. If resistor chips 10 do not change uniformly, gate voltage will vary among the transistors, producing a loss in the intended symmetry. As shown, transistors 18a-18c are disposed on the substrates 10 along the upper part of Figures 14-17. Transistors 18d-18f are disposed on the substrates 10 along the lower part of those Figures. A first plastics spacer 90 is disposed on baseplate surface 82, between the two rows of substrates 10. First plastics spacer 90 is of electrically non-conductive plastics, as is all the rest of the plastics elements to be described in this module. First plastics spacer 90 is a generally plate-like rectangular member that has many added conformations. One of the conformations is an annular wall-like projection 90e on its lower surface. The wall-like projection 90e nests within slot 86. Projection 90e locates first plastics spacer 90 on surface 82, with respect to the elongated rectangular surface portions 84a and 84b. Nested within the wall-like projection 90e and adhesively bonded thereto, is a complex plastics moulding 88. Complex plastics moulding 88 has three cylindrical projections 88a, 88b and 88c, uniformly spaced along the centreline of the length of the slot. Cylindrical projections 88a, 88b and 88c, respectively, have hexagonal recesses 88a', 88b' and 88c', within which clamping nuts 89 are disposed and adhesively bonded. For ease of illustration, the adhesive is not shown. For analogous reasons, it is not shown in any other parts of Figures 14-20 either. The same silicone adhesive as described for the first-described embodiment can also be used for this embodiment. The lower surface of first plastics spacer 90, outside of annular projection 90e, is flat and rests on the underlying portion of baseplate surface 82. The two are adhesively bonded together. First plastics spacer 90 contains a first linear lead frame element 30g and a second linear lead frame element 78g. Both linear lead frame elements are shown in phantom lines in Figure 16. It should be mentioned that Figure 16 also shows a second plastics spacer overlying the first plastics spacer. It is not intended that the phantom lines should indicate that the lead frame is in the second plastics spacer. It is made of record here that nothing is embedded in the second plastics spacer. Hence, it contains no embedded lead frame. The first embedded lead frame element 30g in the first plastics spacer 90 serves as a gate bus for the first group of transistors 18a, 18b and 18c. The second linear bus 78g serves as a second gate bus for the transistors 18d, 18e and 18f. The upper part of spacer 90 is broken away in Figure 14. However, first plastics spacer 90 can be seen in its entirety in Figure 15, along with the exposed parts of the two embedded lead frames. One can see that there are exposed portions of lead frame 30g opposite the resistor chips 32a, 32b and 32c, as well as in extension 90a. The second lead frame 78g is embedded along the opposite edge of first plastics spacer 90. Lead frame 78g has free end portions exposed opposite resistor chips 32d, 32e and 32f. It also has a portion embedded within extension 90b, which is diametrically opposed from the first spacer extension 90a. Spacer 90 has a plurality of additional formations. These additional formations serve to locate subsequently assembled components, such as the terminal plates 34g and 74g and an overlying second plastics spacer 92. Disposed in the same plane on the first plastics spacer 90 are two metal plates 34g and 74g, each of which has a linear edge facing the substrate subassemblies it serves and a large semicircular enlargement. The enlargement on terminal plate 74g is central to the length of the plate. The enlargement on terminal plate 34g is at one end. The two enlargements nest together, separated by a narrow short wall 90c on spacer 90. Part of the enlargement in terminal plate 34g is obscured in Figure 15 by a cylindrical terminal boss 34. Terminal plates 34g and 74g and boss 34 are of nickel-plated copper, as are the other terminal plates 36g and 76g and terminal bosses 36/74 and 76. The terminal bosses 34, 36/74 and 76 each have a central bore to receive a bolt 94. Bolt 94 cooperates with a nut 89 in recess 88a' of plastics moulding 88 to clamp terminal 76g against baseplate 80. As hereinbefore indicated, the clamping nut 89 would be adhesively bonded within the recess 88a'. It should also be noticed that in Figure 15, substrate tabs 14a have now been bent down over their respective terminal plates 34g and 74g. Tabs 14a, therefore have two right angle bends, as does the substrate tab 14a in Figure 1. Each of the portions at tabs 14a that overlie their respective terminal plates 34g and 74g are electron or laser beam welded to those terminal plates. The weld takes the form of two linear weld strips on each tab. However, if desired, tabs 14a can each be soldered to their respective terminal plates 34g and 74g. As previously indicated, the formations on the plastics spacer 90 aid in positioning terminal plate members 34g and 74g with respect to each other and with respect to the tabs 14a. In addition, it should be mentioned that first plastics spacer 90 has an upstanding boss 90d on its left and right edges. These bosses nest in cooperating recesses 92b in the second plastics spacer 92, as can be seen in Figure 16. As can also be seen in Figure 16, second plastics spacer 92 is a generally plate-like rectangular moulded body having alignment conformations. Terminal plates 36g and 76g are disposed on second plastics spacer 92. They are co-planar with each other but aligned over the lower plates 34g and 74g. A short wall 92c helps align terminal plates 36g and 76g over the terminal plates 34g and 76g so that they are in substantial overlapping relationship. The wall 92c in the second plastics spacer 92 is thus analogous to the wall 90c in the first plastics spacer 90. It will be noticed from Figure 16 that the upper pair of terminal planes 36g and 76g are almost mirror images of the lower pair of terminal plates 34g and 74g. However, the upper pair of plates have extensions at their ends that nest in formations on the upper surface of second plastics spacer 92. In addition it should be noticed that the longitudinal edge of each of the upper terminal plates 36g and 76g, has two notches, that are respectively located over exposed portions of the lead frames 30g and 78g in the first plastics spacer. Hence, the notches in terminal plate 36g are opposite resistor chips 32b and 32c. The notches in terminal plate 76g are opposite resistor chips 32e and 32f. In addition, spaced by the notches on the linear edge of terminal plate 36g, are three rectangular aluminium laminations 36g'. Aluminium laminations 36g' enhance bondability of filamentary aluminium wires between terminal member 36g and the respective aluminium metallisations forming gate electrodes on transistors 18a, 18b and 18c. For the same reasons, terminal plate 76g has three rectangular aluminium laminations 76g' along its notched linear edge. The aluminium laminations can be formed in any convenient manner and form no part of this invention. It can be see that the semi-circular enlargement of upper terminal plate 74 overlies the semi-circular enlargement of terminal plate 36. The central bores in these overlapping plate portions are registered. Nickel plated copper washers, of a combined thickness equal to that of the second plastics spacer, are disposed between the overlying semi-circular portions of terminal plates 36g and 74g. Figure 16 further shows cylindrical terminal boss 36/74 in register with these bores and the combination clamped together by a bolt 94 and a nut 89. It will be understood that terminal 36/74 forms a common electrical contact or circuit node, as indicated in the electrical schematics of Figures 12 and 13. Terminal boss 46/74 then forms an emitter terminal for first switch transistors 18a, 18b and 18c and a collector terminal for the second switch transistors 18d, 18e and 18f. The cylindrical terminal boss 76 is similarly clamped to terminal plate 76g, by means of bolt 94 and forms an emitter terminal for the second switch transistors 18d, 18e and 18f. If desired, washers can be used under the terminal plates or under their terminal cylinder bosses to adjust top surface height of the cylinders to match any corresponding connector system that might be employed. In such instance, a common cylinder size could be used for all terminals. It should be mentioned that it may be preferred adhesively to bond the lower pair of coplanar plates to first plastics spacer, adhesively to bond the second plastics spacer to the them and/or the first plastics spacer and adhesively to bond the second pair of coplanar plates to the to second plastics spacer. Alternatively and most preferably, the plates, spacers and washers (if any needed) would all be insert moulded into a single metal/plastics composite part. This would be the simplest construction of all and probably the most economical and reliable. In any event, once the upper coplanar terminal plates 36g and 74g are in place, wire bonding can proceed. Relatively thick filamentary aluminium wire would be thermocompression and/or ultrasonically bonded. The wires would extend between the aluminised portions of the terminal plates and the semiconductor devices on the substrates, as shown. They would also extend from the exposed parts of gate bus 30g to resistor chips 32a, 32b and 32c. They would extend from the exposed parts of gate bus 78g to resistor chips 32d, 32e and 32f. From resistor chips 32a-32f, the aluminium wire would respectively extend to the aluminium-coated gate electrode on transistors 18a-18f. As hereinbefore indicated, relatively thick filamentary aluminium wire is used. However, as shown in Figure 17, multiple strands of such wires is used in the transistor and diode large area contacts, in order to handle the high current at low resistance. The multiplicity of wires on the transistors, as previously indicated also tend to improve fatigue life of the bonds for the wires connected to the transistors. It should be mentioned that geometrical symmetry can be provided even more readily in the linear type of module than in the concentric type of module. It should also be seen that in the linear module the lengths of filamentary wire between the transistors and the terminals extend over tabs 14a. This provides current flow in an opposite but parallel direction. As previously indicated, this tends to negate further input/output inductance. Reference is now made to Figure 17 which shows a housing 96 registered over baseplate 80 and adhesively bonded to baseplate upper surface 82. Housing 96 is of non-conductive plastics and has a ring-like outer portion 96a and a centre bridge portion 96b. The centre bridge portion 96b has three apertures 96c to accommodate the terminal posts, that is bosses 34, 36/74 and 76. The left side of housing ring 96a has an enlargement 96d that contains a recess 96e within which are disposed terminal tabs 30, 36k and 34k. Analogously, the right side of housing ring 96a has an enlargement 96d'. Enlargement 96d' has a recess 96e' within which are exposed terminal tabs 76k, 74k and 78. The terminal tabs 30, 36k and 34k are exposed ends of a lead frame that is embedded within the housing 96 to form electrical connections with various parts of the assembly at different levels and locations beneath the cover 96. The two embedded lead frames are shown in Figure 17 in phantom lines. As can be seen, they are similar in configuration but oppositely disposed. Each lead frame can be made a part of the housing by insert moulding of the housing. In such instance, after moulding, the various interconnecting portions of the lead frame are removed to release the individual parts of the lead frame. The exposed portions can be removed by cutting or stamping, as is usual. In this embodiment, each of the two lead frames includes two webbed portions under the rim 96a. The webbed portions are removed by drilling holes 98 down through the housing plastics and down further through the interconnecting webbed portions. This produces a final separation between the various leads 30, 36k and 34k on the left of the Figure. Analogously, holes 98' are drilled through the right edge of the housing ring 96a down through the connecting webbed portion between terminal members 76k, 74k and 78. The drilled holes separates the lead frame into three discrete electrically isolated terminals. After the holes 98 and 98' have been drilled and the respective electrodes separated, the holes can be filled with any suitable material, as for example the same adhesive used to bond the housing 96 to the baseplate 80. It should also be mentioned that the housing ring portion 96a has holes 100 in its four quarters corresponding to the holes 81 in the four corners of baseplate 80. When cover 96 is bonded to baseplate 80 these holes are left open so that bolts (not shown) can be used to clamp the resultant assembly to a support (not shown). While not previously mentioned, the first plastics spacer 90 has tiny apertures 104 in its opposed extensions 90a and 90b. These apertures extend down into the thickness of plastics spacer 96 and register with corresponding holes in the embedded lead frame assemblies 30g and 78g. It can also be seen in Figure 15 that terminal plates 34g and 74g have similar small apertures. Figure 16 shows small apertures in each of the terminal plates 36g and 76g. Referring now to Figure 17, aperture 102 in the housing 96 intersects an aperture (not separately shown) in first lead frame element 30 that is embedded in the housing 96. Aperture 102 is the same diameter as and registers with the aperture in the first lead frame 30 embedded in housing 96. Aperture 102 is also registered over aperture 104 in lower spacer extension 90b. Tiny aperture 106 in the lower right-hand corner of lead frame 96 intersects an aperture (not separately shown) in second lead frame element 78 that is embedded in the housing 96. Aperture 106 is the same diameter as and registers with, the aperture in the second lead frame 78 embedded in housing 96. Aperture 106 is also registered over aperture is in register with the aperture 105 in the lower spacer extension 90a. Pins (not separately shown) are driven down through each set of registered apertures and through the associated overlapping portions of each pair of embedded lead frames. The pins rectangular in cross-section and hardened, so that they can be can be hammered of pressed down. To ensure a permanent contact, the diagonal dimension of the cross-section of the pins is about 5-25% bigger than the diameter of the apertures. Much bigger than that may make it difficult to drive the pins down into the second lead frame. The length of the pins is slightly longer than will extend from the surface of housing 96 and through the second lead frame (which is embedded in the first plastics spacer. One pin thus provides electrical contact between exposed contact 30 and the gate bus 30g for transistors 18a, 18b and 18c, via apertures 102 and 104. Similarly, a pin driven through apertures 106 and 105 provides an electrical communication between the gate terminal tab 78 and the lead frame 78g for transistors 18d, 18e and 18f. As mentioned above, the chip resistors 32a-32c and 32d-32f were previously connected by filamentary wires to their respective lead frames 30g and 78g and also connected by filamentary wires to the gate electrodes of their respective transistors. Accordingly, the chip resistors 32a-32f can be trimmed so that a predetermined gate potential can be applied to transistors 18a-18f. Presumably, the predetermined voltage is the same voltage for all transistors. However, it should be recognised that equal potential is intended to provide equal performance. It is conceivable that in some instances a different potential may be desired for some or all of the transistors 18a-18f in order to obtain equal performance. For example a given transistor may need more gate voltage to reach the same operating level as the other transistors. Kelvin connections have not yet been described for this embodiment. They are provided from the housing lead frame using a driven-pin type of connection, such as described in the preceding paragraph. They are provided by driving a pin, that is rectangular in cross-section and hardened, into the other apertures in the housing. This connects selected discrete members of the housing leads to selected terminal members. For example, aperture 108 extends through a housing lead frame member and is registered over an aperture terminal plate 36g. Driving the aforementioned type of pin down through these apertures will interconnect terminal tab 34k with terminal plate 34g. A pin driven down through aperture 110, connects contact tab with terminal plate 36g. A pin driven down through aperture 112, connects contact tab 74k with terminal plate 74g. A pin driven down through aperture 114 connects contact tab 76k with terminal plate 76g. Once the pins have all been driven in, the apertures can be sealed. Adhesive can be used. Then, the chamber formed by housing 96 and baseplate 80 can be filled with a silicone resin or oil, if desired. The filling is not shown because it is optional and well known. A cover 116, made of non-conductive plastics or the like is then placed over the housing 96. The outer periphery of cover 116 nests within a circumferential groove in housing 96. An adhesive (not shown) is disposed in the circumferential groove 118 to bond cover 116 to housing 96. Housing 116 has apertures 116a corresponding to and in register with the apertures 96c in the housing bridge portion 96b. Apertures 116a are in register with the apertures 96c in the bridge portion 96b of housing 96 to accommodate the terminal bosses 34, 36/74 and 76. Figures 18 and 19 are respectively width and length centreline cross-sectional views of the completed device. To emphasise better the difference between plastics and metal in these views, plastics is shown with traditional cross-sectioning. Metal is not. In addition, some of the metal elements are shown in elevation. For example, the clamping nut 39 and bolts 94 are shown in elevation in Figures 18 and 19. Baseplate 80 is the same composite material as used for baseplate 62 in the single switch of Figure 3-4 and 6-11. However, in this embodiment baseplate 80 is hollow. In addition, its interior contains cylindrical post-portions 118 extending between upper and lower interior surfaces of the hollow baseplate 80. The interior posts 118 are disposed in a uniform array to enhance heat transfer between substrate 80 and a coolant fluid that passed through the baseplate 80. The cooling fluid enters the interior 122 of baseplate 118 through an entrance duct 120. Interior 122 is at one end of the length of the baseplate and interconnects with two interior portions 124 of the baseplate. Interior portions 124 are disposed on either side of slot 86. The other ends of baseplate interior portions 124 communicate with the interior portion 126 at the other end of the baseplate. Interior portion 126 connects with an outlet port 128. As previously mentioned, it is important that cooling of all the devices be similar to obtain similarity in electrical operation. Accordingly, the design and arrangement of the columns 118 is the same on both sides of the baseplate aperture 86. Similarly, the flow of cooling fluid from inlet 120 to outlet 128 is designed to be similar on both sides of slot 86. If the cooling is similar, electrical performance of the devices will more likely be similar. Also, it can be seen in Figure 17 that each of the exposed electrode tabs 30, 36k, 34k, 74k, 76k and 78 have small apertures in their exposed ends. Pin connectors having a rectangular cross-section can be driven through these apertures to make a permanent connection between them. On the other hand, short pins may be permanently driven into these apertures, with the pins themselves becoming a male connector pin for a push-on female connector. Other aspects of the present invention are covered in our co-pending European patent application no EP-A-0645814 filed the same day as this application.

Claims: A high power/high frequency transistor module including a terminal member (34) and at leat one transistor substrate subassembly (10), said transistor substrate subassembly comprising a ceramic wafer (12) having first and second opposed major surfaces and a single high power/high frequency switching transistor chip (18), wherein first and second metal foil layers (14, 16) bonded to said ceramic wafer (12) cover substantially the entirety of said first and second major surfaces of said ceramic wafer (12), and the chip (18) is conductively bonded to said first metal foil layer (14); characterized in that said first metal foil layer (14) includes an integral portion that extends outwardly from said wafer (12) forming a tab (14a) and interconnecting a region of said chip (18) to said terminal member (34). A high power/high frequency transistor module according to claim 1, in which the ceramic wafer (12) is of beryllium oxide. A high power/high frequency transistor module according to claim 1 or claim 2, in which said first metal foil layer (14) is copper, said chip (18) is an insulated gate bipolar switching transistor chip, and a collector region of said chip (18) is soldered to said first foil layer (14) so that said tab (14a) interconnects said collector region to said terminal member (34). A high power/high frequency transistor module according to any one of claims 1 to 3, comprising a thermally conductive baseplate (44), a plurality of said transistor substrate subassemblies (10) on said baseplate (44) such that the tab (14a) of each such subassembly (10) is in substantially equal proximity to said terminal member (34), a thermally conductive bond between said subassemblies (10) and said baseplate (44), and an electrically conductive bond between each subassembly tab (14a) and said terminal member (34). A high power/high frequency transistor module according to claim 4, in which welds (52) bond the subassembly tabs (14a) to the terminal member (34). A high power/high frequency transistor module according to claim 4 or claim 5 in which the first and second metal foil layers (14, 16) of each subassembly (10) are copper having a coating of silver to enhance solderability, and in each subassembly (10), said chip (18) is soldered to said first metal foil layer (14), said second metal foil layer (16) is soldered to said baseplate (44).

Inventors: ALHOUSSAMI AIMAN ISMAIL, EYTCHESON CHARLES TYLER, LAKE DONALD EUGENE, ALHOUSSAMI, AIMAN ISMAIL, EYTCHESON, CHARLES TYLER, LAKE, DONALD EUGENE
Applicants: DELCO ELECTRONICS CORP, DELCO ELECTRONICS CORPORATION
Application year: 1994
Main classification: H01L23
IPC codes: H01L25, H02M7, H01L23